Electrical cooker unit for a ceramic glass plate type electrical cooker

ABSTRACT

An electrical cooker unit for use in the type of cooker in which a ceramic glass plate is placed above the electrical heating elements. The cooker unit consists of a cylindrical metal tray, the base and side wall of which is lined with electrical and thermal insulating material formed from particulate microporous thermal insulating material which has been consolidated into block form. The base layer of insulating material has a flat planar upper surface and the electrical heating element consists of a helically coiled uncovered wire laid out in the form of a flat spiral on such upper surface and secured solely by metal staples which engage in the base layer for part of the depth thereof. The ceramic glass plate rests upon the upper edge of the side wall of the thermal insulation, leaving a space between the under side of the plate and the electrical element.

United States Patent 11 1 1111 3,833,793 McWilliams et al. 1 Sept. 3, 1974 [5 ELECTRICAL COOKER UNIT FOR A 3,612,828 10 1971 Siegla 219/464 E IC GLASS PLATE TYPE 3,612,829 10/1971 Evans et al. 219/464 3,624,352 11/1971 Deaton et al. 219/449 ELECTRICAL COOKER 3,646,321 2/1972 Siegla 219/464 [76] Inventors: Joseph Anthony McWilliams, 160 3,710,076 1/1973 Frazier 219/449 Sutton Park John Thomas 3,733,462 3/1973 Bouchard 81, al. 219/464 Hughes, 3 Moorfields, Mamble, both of Kidderminster, Primary ExaminerVolodymyr Y. Mayewsky Worcestershire, England Attorney, Agent, or Firm-Holman & Stern [22] Filed: Aug. 3, 1973 21 Appl. NO.I 385,558 [57] ABSTRACT An electrical cooker unit for use in the type of cooker in which a ceramic glass plate is placed above the [30] Foreign Apphcatlmi Priority Data electrical heating elements. The cooker unit consists Aug. 5, 1972 Great Br1ta1n 36682/72 of a cylindrical metal tray, the base and Side wall of which is lined with electrical and thermal insulating [52] US. Cl 219/464, 219/460, 219/463, material formed from particulate microporous thermal 219/467 219/459 insulating material which has been consolidated into [51] [1.1L Cl. H05b 3/68 block form. The base layer of insulatin material has a 58 F 1d 1 S h 219/449 450 455 459 g 1 0 care flat planar upper surface and the electrical heating'el- 219/460 467 ement consists of a helically coiled uncovered wire laid out in the form of a flat spiral on such upper sur- [56] References C'ted face and secured solely by metal staples which engage UNITED STATES PATENTS in the base layer for part of the depth thereof. The ce- 3,086,101 4/ 1963 Scofield 219/464 X ramic glass plate rests upon the upper edge of the side 3,496,336 2/1970 Hingorany et al. 219/464 wall of the thermal insulation, leaving a space between 3,500,444 0 Hesse et a 219/464 X the under side of the plate and the electrical element. 3,567,906 3/1971 Hurko 219/464 3,612,827 10/1971 Dills 219/463 3 Claims, 2 Drawing Figures *18 13 17 g :3. 371 7 1:.- i 1 i A :l 11" 3;, t 1: I l: 1 I 1. 1, =14 4116216211 ;412x2 1 41 1 441 44:11:42: "21 2' 212 54 1 i4 to:

PATENTEU SEP 1974 O MQQ Q FIGZ.

ELECTRICAL COOKER UNIT FOR A CERAMIC GLASS PLATE TYPE ELECTRICAL COOKER BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to an electrical cooker unit for use with the type of electrical cooker in which a ceramic glass plate fonns the actual heating surface upon which a cooking utensil is placed and the cooker unit proper which includes the electrical heating element being disposed below the glass plate.

The cooker unit itself comprises a metal supporting tray the inside of the base and side walls of which is lined with thermal insulating material and the electrical heating element being supported upon the base of insulating material. One or more of such cooker units may be incorporated in a complete electrical cooker and the ceramic glass plate (or plates) is arranged to engage the upper edges of the side wall thermal insulating material so that the under surface of the ceramic glass plate is spaced away from the electrical heating element.

2. Description of the Prior Art As available hitherto, such ceramic glass plate electrical cookers have a poor performance as compared with move conventional cookers in which the electrical heating element is not covered and wherein the heating of a cooking utensil is by direct radiation or direct contact with the heating element, being slower and less responsive to temperature control adjustments.

The sheathed type of electrical heating element when used with ceramic glass plate covering has very poor performance and the tendency has been hitherto towards using an exposed wire type of heating element which relies solely upon direct radiation for its heating performance. However, such exposed wire type of heating element, when used with ceramic glass plate cookers has not been able to achieve satisfactory heating performance comparable with the more conventional type of electrical cooker using elements which are not covered and it has been found that this is due to the manner in which the wire heating element is supported upon the layer of thermal insulation material in the base of the supporting tray.

As used hitherto, the exposed wire heating element has comprised a helical coil which has been laid in a groove formed in the layer of insulating material in the base of the tray. Generally, the helical coil wire is laid in the form of a flat spiral and the supporting tray has been of cylindrical form. The groove in the layer of insulating material is of part-circular form and in one type used hitherto the helical coiled wire is cemented into the base of the groove so that only part of the surface of the wire is exposed above the level of the insulating material and the part of the wire in the base of the groove is embedded in cement. In this case the cement and surrounding insulation in the groove provide a high thermal mass and a substantial amount of heat from the wire is wasted in heating up this mass. In other cases the coil is laid in a part-circular groove without cementing but in this case there is a substantial amount of heat loss by direct radiation from the wire into the Walls of the groove and a good deal of useful heat is lost in heating up thehigh thermal mass provided by the walls of the groove. 7

The object of the invention is to provide an improved form of electrical cooker unit for use with ceramic glass plate type electrical cookers which will avoid the above mentioned disadvantages and provide a ceramic glass type electrical cooker with a heating performance comparable to that of more conventional electrical cookers having exposed heating elements.

SUMMARY OF THE INVENTION According to the invention there is provided an electrical cooker unit for a ceramic glass plate-type electrical cooker, such unit comprising a base layer of thermal and electrical insulating material including a flat planar supporting surface for the electrical heating element which is in the form of a helically coiled uncovered wire and is secured to the supporting surface by metal wire staples which engage over parts of the wire convolutions where these engage the supporting surface and which pass into, but not entirely through, the base layer, the composition of the material of the base layer being such that the heating element is held in position solely by virtue of the frictional grip of such material upon the staples.

A cooker unit made in accordance with the invention has the following important technical advances as compared with cooker units used hitherto and as hereinbefore referred to. 7

Because the wire of the heating element is supported by a flat planar surface there is only minimal loss of available heat to the thermal insulating material and substantially the whole of the radiant heat is available for useful work.

Only an insignificant amount of heat is lost through conduction via the staples into the base layer of thermal insulating material due to the low mass of the staples and the fact that each staple makes contact with the wire of the heating element only over a very minute surface area.

Because they are electrically conducting the metal staples must not make contact with the metal of the supporting tray and must, therefore, terminate a safe distance short of the base of the tray. Thus, for their holding power the metal staples must rely wholly upon the frictional grip of the thermal insulating material in which they are embedded and this leads to the necessity for careful choice for the material of the thermal insulation in order to ensure that the composition thereof will provide the necessary frictional grip. A preferred form of material for the base layer of thermal insulation is that formed from particulate microporous thermal insulating material (such as microporous silica aerogel) which has been consolidated under pressure into the form of a block or slab, the pressure causing the particles of microporous material to bond together.

Where, as is most usual, the helical wire coil forming the heating element is laid out in the form of a flat spiral the staples in adjacent convolutions of the spiral are preferably staggered in relation to one another so as to avoid the possibility of the legs of one staple making contact with the legs of an adjacent staple.

Preferably, the staples are of inverted V or hairpin form and formed of a high temperature resistant metal such as a nickel chrome alloy capable of withstanding temperatures up to 900C or more.

A preferred thermal and electrical insulating material for the base layer and for lining the side walls of the tray has a microporous structure and is made by the bonding of an intimate mixture of a microporous powder with an opacifier and a reinforcing fibre.

The microporous powder is preferably finely divided silica for example microporous silica aerogel which is a gel in which the liquid phase has been replaced by a gaseous phase in such a way as to avoid the shrinkage which would occur if the gel had been dried directly from a liquid. A substantially identical microporous silica structure can be obtained by controlled precipitation from a silica solution, the temperature and'pH being controlled during precipitation to obtain an open structured precipitate. Similar products can be obtained by pyrogenic or electrothermal operations and will be suitable for use if the average ultimate particle size is less than 100 milli-microns.

The opacifier used in the preferred insulating mate- BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a plan view from above of a cooker unit, and

FIG. 2 is a section on line 2-2 of FIG. 1 showing a ceramic glass plate in chain-dotted outline.

DESCRIPTION OF THE PREFERRED EMBODIMENT With reference to the drawings, the outer metal tray of the apparatus is of dish-like form having a base and a side wall 11 and the electrical heating element 13 rests upon a thin disc 14 of thermal insulating material which itself is supported upon the base layer 15 of microporous insulation. The disc 14 is supported upon the base layer 15 of microporous insulation. The disc 14 is preferably a bonded ceramic fibre composite and provides a good fiat supporting surface for the element 13.

The layer 15 comprises the base of a dish-shaped piece of thermal insulating material formed of microporous insulation and provides an upstanding side wall 16 which is in contact with the interior of the side wall 11 of the outer tray.

Preferably, the microporous insulation 15, 16 would be moulded into the outer tray and would be adequately held in position by the friction between its own surface and the interior surface of the tray. Alternatively, it maybe machined from a block of microporous insulating material making the external dimensions slightly oversize so that'it can be forced into the outer tray and will be retained by the frictional engagement between its outer surface and the inner'surface of the tray.

A thin section ring 17 of thermal insulating material of the same composition as the disc 14 rests upon the upperedge of the wall 16 and is secured to the wall 16 by suitable means such as a number of pins or staples (not shown). The glass ceramic cover 18 is mounted directly upon the ring 17.

The electrical heating element 13 is secured in position by means of a number of U-shaped staples 19 which are driven through the disc 14 and into the layer 15 of microporous insulation which has a high coefficient of friction so as to provide good resistance to withdrawal of the staple. Also, the insulation material has a good deal of resilience and when the staple is driven through into the material local stresses are created therein which have the effect of applying a pressure to the legs of the staple to resist its withdrawal. As will be seen, the staples terminate short of the base 10 of the metal tray.

The staples are spaced apart around the spiral of the heating element as illustrated by the marked positions of staples 19 in FIG. 1 and it will be seen that the staples are staggered in adjacent convolution.

The connection of an electrical supply .lead to the inner end of the wire of the heating element is shown in section in FIG. 2, the connection of the outer end of the wire to an electrical supply lead being exactly the same. The lead to the electrical supply passes through a sheath 21 of electrical and thermal insulating material the sheath 31 being secured in a flanged sleeve 22 where it passes through the base 10 of the tray. Within the sheath 21 the wire 20 coming from the heating element is braided with one or more further wires as indicated at 23 to increase the electrical resistance and at the position where the one or more further wires are joined to the wire 20 the collection of wires is crimped inside a flat metal sleeve. This form of connection is considered to be advantageous over a brazed or welded connection as it is less liable to embrittlement failure under high temperatures.

It will be observed also that each staple 19 engages the wire at the base of the coil of the helix, i.e. where the wire contacts the disc 14.

We claim:

1. An electrical cooker unit for a ceramic glass plate type electrical cooker, said cooker unit comprising a thermal insulating base layer formed by bonding an intimate mixture of particulate microporous silica aerogel of average particle size less than millimicrons with an opacifier and reinforcing fibers, said base layer having an upper surface which lies in a flat horizontal plane, an upstanding side wall of thermal insulating material surrounding said base layer and having an upper edge lying in a horizontal plane spaced above the plane of the base layer, a layer of ceramic fibre composite of substantially less thickness than the thickness of the base layer is provided on the upper surface of the base layer and on the upper edge of the side wall, an electrical heating element comprising bare wire in helically coiled form supported upon the upper surface of the.

ceramic fibre composite layer on said base layer with the convolutions of the helical coil standing upright in substantially vertical planes and the whole of the heating element being spaced below the plane of the upper edge of said side wall, and a plurality of metal wire staples each of inverted V-form as the sole means for securing said heating element in position, each staple being of effective length less than the depth of the base layer and being driven through the ceramic fibre composite layer and into the base layer so that the apex of its V-form engages over the lower portion of a convolution of the wire at the position where the convolution of the base layer also extending upwardly therefrom to provide the said side wall which is in engagement with the interior of the outer wall of the tray.

3. A cooker unit according to claim 1 wherein the ends of the wire of the heating element pass downwardly through the base layer and are electrically insulated and cripmed in metal sleeves to further wires to form the electrical supply leads. 

1. An electrical cooker unit for a ceramic glass plate type electrical cooker, said cooker unit comprising a thermal insulating base layer formed by bonding an intimate mixture of particulate microporous silica aerogel of average particle size less than 100 millimicrons with an opacifier and reinforcing fibers, said base layer having an upper surface which lies in a flat horizontal plane, an upstanding side wall of thermal insulating material surrounding said base layer and having an upper edge lying in a horizontal plane spaced above the plane of the base layer, a layer of ceramic fibre composite of substantially less thickness than the thickness of the base layer is provided on the upper surface of the base layer and on the upper edge of the side wall, an electrical heating element comprising bare wire in helically coiled form supported upon the upper surface of the ceramic fibre composite layer on said base layer with the convolutions of the helical coil standing upright in substantially vertical planes and the whole of the heating element being spaced below the plane of the upper edge of said side wall, and a plurality of metal wire staples each of inverted V-form as the sole means for securing said heating element in position, each staple being of effective length less than the depth of the base layer and being driven through the ceramic fibre composite layer and into the base layer so that the apex of its V-form engages over the lower portion of a convolution of the wire at the position where the convolution engages the upper surface of the base layer, the microporous material of the base layer having a high coefficient of resistance and exhibiting resilience whereby the driving of the metal staples into the base layer results in local reaction forces being created in the material in the region of the legs of the staples and acting thereon to prevent withdrawal.
 2. A cooker unit according to claim 1 including an outer metal tray having a base, which supports the said base layer, and a surrounding outer wall, the material of the base layer also extending upwardly therefrom to provide the said side wall which is in engagement with the interior of the outer wall of the tray.
 3. A cooker unit according to claim 1 wherein the ends of the wire of the heating element pass downwardly through the base layer and are electrically insulated and cripmed in metal sleeves to further wires to form the electrical supply leads. 